What are the differences between primary and secondary succession?

Primary succession occurs in places where a community has not already been established, such as land where glaciers are receding.  Secondary succession occurs in places where the establish community has been damaged or destroyed by things such as floods, fire or logging.

Primary succession involves the invasion of living organisms in an area that had no previous existing community or soil. Soil forms from the erosion of rock material in which plants can establish and spread throughout the ecosystem. Over time, a climax community forms complete with plants, animals, fungi, and bacterial species. Secondary succession occurs when an established ecosystem is cleared either by human activity or natural processes. Secondary succession involves opportunistic species such as grasses, weeds, and shrubs that invade and establish. If the ecosystem is not disturbed, then these plants will eventually be out-competed by other plants.